10 things to know about the move to IPv6

The Internet has been rolling along for decades on the strength of IPv4 and its numbering system, which has supplied billions of addresses. As long as more addresses were available, few people thought about them. But the booming popularity of the Internet has finally soaked up nearly all those fresh numbers: In February, the Internet Corporation for Assigned Names and Numbers (ICANN) allocated the last of its unused large blocks of IPv4 addresses to regional Internet registries. On Wednesday, World IPv6 Day will turn the new protocol on at hundreds of companies, agencies and universities for testing. Suddenly, IT administrators and consumers alike are starting to think more about IP addresses. Here are the answers to a few questions about the numbers that make the Internet work.

1. Why do we need IP addresses?

Just as every letter sent through the postal service needs addresses to show both where it's going and where it's coming from, every packet on the Internet needs two IP addresses: destination and source. Those addresses direct packets to and from PCs, servers, and virtual machines. Each of those kinds of machines may have multiple IP addresses, some public and some just for use on the local network.

2. What do IP addresses do for me?

IP addresses are used for many Internet applications, some of them invisible to users, including machine-to-machine communications. Even in the simplest example, Web browsing, there are several steps. When you type in the URL for a website, your browser calls a piece of software on your device, the DNS (Domain Name System) resolver, and tells it to contact a name server, which may exist on your corporate LAN or at your ISP. The DNS resolver asks the name server to find an IP address that is assigned to the domain you typed in. If it finds one, it sends that address back to the browser, which uses the address to take you to the right domain.

3. How many IPv4 addresses are there?

There are about 4 billion IPv4 addresses, because the length of the addresses in binary form -- 32 bits -- allows for that many possible unique combinations. IPv6 addresses are 128 bits long, so there are many more unique sequences of numbers that can be created for them: specifically, 3.4 times 10 to the power of 38. This is also known as 340 undecillion. The word undecillion designates a number with 11 sets of three zeros, plus one more set in the numbering system used in the U.S. and many other countries. So the number is rendered as 340,000,000,000,000,000,000,000,000,000,000,000,000. Most experts believe that 340 undecillion addresses is essentially an inexhaustible supply.

4. Am I likely to get stuck without an IPv4 address?

If you are a mobile Internet subscriber in Asia, you may pretty soon, according to Cricket Liu, vice president of architecture at Internet software vendor InfoBlox. APNIC (Asia-Pacific Network Information Centre) kicked off the final allocation of IPv4 addresses to the regional registries because it needed two new blocks of addresses to satisfy fast-growing demand in the region. Mobile phones are the first Internet devices for many people in developing parts of Asia, and assigning IP addresses to those is part of the reason for the crunch. When mobile operators run out of IPv4 addresses, they will have to resort to selling phones with only an IPv6 address, Liu said.

5. What would happen if I only had IPv6 and tried to go to an IPv4-only website?

In the worst case, you wouldn't even get the familiar "server not found" message, according to Liu. This type of warning, also known as a "404" message, comes from the Web server of an existing domain that no longer hosts the page you're looking for. Without a usable IP address, no Web server can be reached. "It would probably just give you some sort of a network error," Liu said. Leo Vegoda, manager of number resources at the ICANN, said the message you receive will be up to your browser vendor.

6. So, would IPv6 cut me off from the Web?

In reality, you would probably get to see the website anyway, according to Liu of InfoBlox. Service providers are evaluating a variety of tools for delivering IPv4-only content to IPv6-only clients. The most common one is a network address translation technique called NAT64, which can run on an appliance attached to a service provider's network. When a name server can't find an IPv6 address associated with the website that the user wants to visit, a NAT64 appliance can take the host's IPv4 address and encapsulate it within an IPv6 address, creating something that the IPv6-only client can understand. Similarly, that appliance could allow users with different types of IP addresses to send e-mail to each other.

7. What if a site goes IPv6 and I'm still on IPv4?

That's not likely to be a problem, at least in the next few years, because when companies adopt IPv6, most of them will use a "dual-stack" configuration, experts said. Dual network stacks contain all the software needed for communication with both IPv4 and IPv6 systems. When a client requests an IP address of either kind that's associated with a given domain, they can get one. This is the system that's really in place at most organizations that are using IPv6 today. "It's not common to support v6, but if you do support v6, then it's very common to run dual stack," Liu said.

One ISP, Hurricane Electric, plans to offer NAT64 as a managed service, with the IPv6 address residing in its own data centers. Enterprises will be able to run their own servers on IPv4 and rely on Hurricane to link them to IPv6 users, according to Martin Levy, Hurricane's director of IPv6 strategy. All the enterprises will need to do is add an entry to their DNS server, he said. Some Hurricane clients are already using the service in early release and will be using it on World IPv6 Day on Wednesday.

8. How long can I keep my company on IPv4?

As the supply of new, unique IPv4 addresses dwindles, some enterprises and service providers will use traditional NAT (network address translation) to conserve the addresses they already have. This technique, already used in everything from corporate LANs to home broadband routers, allows multiple clients on an internal network to share one or a few unique IPv4 addresses to talk to the Internet. Each shared IP address could keep as many as a few hundred systems chugging along without the need for new IPv4 or IPv6 addresses. In any case, IPv4 and IPv6 are expected to coexist on the Internet for many years to come.

9. So why don't we just use NAT instead of making people migrate?

NAT typically uses a stateful firewall, which has to maintain information about the ongoing Internet sessions that all the systems sharing an address are using, ICANN's Vegoda said. The capacity of the appliance limits how many sessions it can keep track of, and if it reaches the limit, long-running sessions may be dropped. While this might not affect a five-minute YouTube video, it could cause problems for watching a full-length movie. In addition, some Internet applications use many sessions at once. For example, an online map may use a separate session for every tile of the map image, Vegoda said. If any of those sessions is broken, it could delay completing the map.

NAT64 doesn't have to maintain state information. It is fundamentally different from traditional NAT and really shouldn't even share its name, according to Hurricane Electric's Levy. But even that technology can be a bottleneck. "If you put a box between source and destination of any variety that is interpreting the packets, you are going to affect the quality of the communication from one end to the other," Levy said.

10. What will World IPv6 Day prove?

Google, Facebook, Yahoo and 315 other companies, agencies and universities will provide Web content over both IPv6 and IPv4 for 24 hours. This will allow people who have IPv6 clients to use them to access many different sites. The idea is to expose any technical problems, such as misconfigurations in end-user equipment or software problems at carriers, that could keep users from getting on the Internet using the new protocol.

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